Tube for boilers, heat exchangers, and the like



NOVe 5, 1934. R 5 BROWN 1,979,859

TUBE FOR BOILERS, HEAT EXCHANGERS, AND THE LIKE Filed Aug. 29, 1932 2 Sheets-Sheet l R. 5. BROWN Nov. 6, 1 934.

TUBE FUR BOILERS, HEAT EXCHANGERS, AND THE LIKE Filed Aug. 29, 1932 2 Sheets-Sheet 2 Patented Nov. 6, 1954 UNITED STATES PATEN OFFICE TUBE FOR BOILERS, HEAT EXCHANGERS, AND THE LIKE Roger Stuart Brown, Chicago, Ill.

Application August 29,

10 Claims.

5 tively low pressure' It will be realized that I do not wish to limit myself, except so far as set out specifically in the claims, to any particular use of my invention, it being adaptable to a wide variety of uses.

One object of the invention is to obtain a greater heat transfer per square foot of tube surface, and a greater number of square feet of surface per lineal foot of tube of a given width, thereby obtaining a greater capacity from apparatus of a given size, under a given allowable pressure drop for the gases between which heat is exchanged. Another object is to lessen or minimize draft resistance, for example, by streamlining the exterior of the tubes, or by eliminating tortuosities of the passage or path traveled by the gases. For example, by streamlining the exterior of tubes at a given velocity of the medium, for instace furnace gases, such medium is circulated across a bank of tubes at more uniform speed and with elimination of back eddies, and consequently less draft loss. A further object is lessening the tendency of soot to deposit in or on, tubes by the elimination of dead spaces in the flow. Various other objects of my invention will appear from time to time in the course of the specification and claims.

I illustrate my invention more or less diagrammatically in the accompanying drawings, where- Figure 1 is a transverse section through a bank of tubes;

Figure 2 is a similar section illustrating a variant form of tube;

Figure 3 is a side elevation and partial section of the tubes shown in Figure 1;

Figure 4 is the end elevation of one of the tubes of Figure 3;

Figure 5 is the side elevation and partial section of one of the tubes with transverse ribs shownlin Figure 2;

Figure 6 is a section on the line 66 of Fig- 1932, Serial No. 630,847

Like parts are indicated by like symbols throughout the specifications and drawings.

Referring generally to the drawings, I illustrate a bank of tubes for example, for a boiler air heater, so arranged, and. the exterior being so shaped, as to provide passages of generally constant cross sectional area between the tubes,

with passage turns limited to a minimum and with the elimination of projections, bends and sharp turns, which tend to cause eddies in the circulation of the exterior medium. Staggering of tubes across the flow of the heating medium increases resistance to the flow, but such staggering is often helpful, in particular to bring the gases circulating outside of the tube in contact with as great a percentage of its periphery as possible. It is to minimize the results of such staggering that I have developed the streamlining and uniformity of cross sections in the passages above referred to.

Referring for example to Figure 1, I illustrate tubes A A, each such tube having diametrically opposed points or elongations A A which are streamlined as shown in the figure. The tube is so shaped that the tubes may nearly touch at the maximum diameter, or they,may be separated by a small space as shown. Note that the outside of the tube is so formed and the tubes are so spaced, that passages A are formed around them which are substantially of equal width, and the passages progress in a series of gently flowing arcs. Note also that the interior A of the tube substantially follows the outside outline, that is, the wall is of substantially uniform thickness. In Figure 3 is shown a side elevation and partial axial section of two of the tubes as grouped in Figure 1, the end view being shown in Figure 4. By reversing the large and small ends of the tube as shown in Figure 3, it is possible to group them closer. The holes in the far end plate are shown by the circles in Figure 1. The tubes in each row are placed with the large ends one way, and the tubes in the next row with the large ends the other way. Figure 3 shows one end of the tube forced back into a round shape at A The other end is first forced back into the round shape as section A whose outside diameter is slightly greater than the maximum diameter of a streamlined portion A A This permits withdrawing the tube through the hole in the header into which A is rolled or welded.

However where it is not desired to roll tubes into the header I may leave the tube section constant throughout, and weld them into holes of simat A and then expanded into a larger round ilar shape in a header sheet. This permits of closer spacing with greater heat transfer.

While I have shown the large round end A on the tube of Figure 3 as made integral with the tube, I may use a separate piece, as for instance is shown in Figure 3 of my co-pending application No. 466,755, filed July 9, 1930, which has since matured into Patent No. 1,870,760, dated August 9, 1932. This is particularly valuable in connection with very thin-walled tubes which cannot stand the additional stretching to expand themout to a larger diameter. The ends A and A may be rolled into grooved holes in headers or return fittings, or welded in, or otherwise suitably fastened (see Fig. 3). I do not wish to be limited to any specific means for securing the tubes to the header, but I do find the means herein' shown practical and eflicient.

Where the sides of the tube in section define the arc of a circle, I find it desirable to use the center of this circle to strike the concave are defining the section of the concave sided streamline extension opposite. The length of the radius of the concave sides of the streamline extension will be the radius of the side of the tube plus the width of the passage.

Figure 2 shows a group of tubes of similar cross section of the body of tube but with a series of parallel fins A shrunk, pressed, or welded to the tube. They may also be applied by expanding the tube over a mandrel against the hole in the fins, or by brazing, or in any other way so that the fins are applied to the tubes in an effective heat-conducting relationship, or where a cast tube isadvantageous they may be integrally cast. Note that this transverse fin extends out from the tube approximately an equal distance at all points. That is, the periphery follows the general shape of the exterior of the tube. I find it convenient to cut off the top and bottom straight across at A -As shown in Figure 2, these fins extend out from the tubes from each side nearly across the space between. Clearance around the fins A may be left for the intermediate and end supporting plate or grating. By keeping the height of the fin constant, I achieve the maximum heating effect without getting any one part of the fin too far away and, in certain cases, therefore, too much hotter than the wall of the tube. These fins not only serve to greatly 'add to the heat-conducting effect on the outside of the tube, but they greatly help to keep the tube in shape, and in particular from bursting out of shape under even low pressures of the medium inside.

In Figure 5 I show a side view and section of the tubes grouped in Figure 2, showing how they rest on the fins in an end supporting plate or grate B, which is shown in front elevation in Figure 9. Figure 6 is a section on line 6-6, of Figure 5.

For high temperature work it is desirable that tubes be made seamless. With tubes of this design it is possible to start with a seamless round tube and deform it to shape shown by pressing, rolling, or drawing over a mandrel. In Figure 6 note that the inside 'of the streamline extensions A are not pressed tightly together, but a small space A with a small radius at the bottom is left between the sides of each streamlining extension. This serves a twofold purpose. It prevents cracking of the tube wall where it is bent sharply around this point, and it also in high temperature work has the great advantage of preventing the tip of this streamlined portion from being overheated and causing burning'or warping due to the unequal expansion. For example, if the cooler medium is inside of the tube, some of it will circulate through the space A keeping the tip temperature down below the point of deterioration, which would not otherwise be the case.

When the tubes are made with transverse fins as at A in Figure 5, they should be parallel to the -fiow of the gas flowing around the outside, and not necessarily perpendicular to the axis of the tube as shown.

For very low pressures, as for instance several inches of water, I find the form of tube shown in Figure 8 to be economical. For such pressure a very thin wall is necessary, compared to the diameter of the tube. It is difficult and expensive to make such a round tube, and I may, therefore, make the tubes by pressing or rolling two fiat sheets into the characteristic form and fasten the edges A together, preferably by fusion, gas, or arc welding, though other methods such as copper hydrogen brazing may be used for some purposes. With such design of tubes, the trans verse ribs A of an approximately equal heightaround the tube are particularly valuable in maintaining its shape.

It will be realized that whereas I have described and shown a practical and operative device, and have illustrated various modifications and applications thereof, that I do not wish to be limited specifically to the forms herein described and shown, except so far as I limit myself by the 9 language of my claims. I wish my description and drawings to be taken in a broad sense, illustrative and diagrammatic rather than as limiting me precisely to-the details indicated. The use and operation of my invention are as follows:

In current practice in the design of such low pressure heat exchangers as air heaters, round tubes are used arranged in a bank with rows staggered across the direction of fiow of the outer medium, usually by one-half .the tube spacing across the direction of flow. In this arrangement, the gas streams passing through the bank find a minimum width of passage where adjacent tubes come closest together. Once past this point, the passage widens out, tending toreduce the velocity of the stream, and causing back eddies in back of the tubes. ,This causes loss of velocity head. One phase of my invention is the provision of a shape of tube whose surface confines the gases, as they pass between the tubes to passages of relatively constant cross-sectional area where the tubes are staggered. I obtain this result by symmetrically streamlining the tubes in cross-section and by arranging tubes so streamlined in such fashion that the distance between the opposed tubes is constant; the result is an easy passage for the gases outside of the tubes along a series or successionof flowing curves without sharp points or angles, or restric tions in the passage or expansion chambers, or any means or method likely to delay or impede the fiow of the exterior circulating medium.

Stated generally, by the various forms of tubes, I illustrate, I obtain a uniform channel for the heated gases, a relatively smooth passageway of constant cross section between the tubes. I therefore obtain a minimum interference with the flow of the gases, and a minimum loss of velocity as the gas around the tubes passes each row of tubes. At speeds below a few feet a secand, gases will travel in a'stratified flow, and it is helpful to baflle or stir up the flow. But for most commercial air heaters and the like.

speedsgreater than this are used and there is constant turbulence and flow within the medium and no necessity to agitate it since it provides its own turbulence. 'Instead of disturbing the flow by changing the width of the channel, it is my idea to disturb it as little as possible, and so obtain the minimum draft loss for a given velocity.

When I speak of gases, I mean it to be taken in a broad sense, gaseous as distinctive from liquid. Thus, it may be'rnixed gases of combustion or it may be air. Where one of the mediums to be heated or cooled is'a liquid, I put it inside the tube. Much greater total heat transfer can be obtained by streamlining the gas passages to form sinuous equal width passages, than by streamlining water passages. There is a; very high film heat transfer on the water side. Increasing that by streamlining adds only a fraction to overall heat transfer compared'to that possible by streamlining and thereby increasing with a given draft the velocity in the gas passages.

In the use of my invention by reducing the loss of velocity head between rows of tubes it is possible to build a unit of the same heat transferring capacity with a. lower draft loss for a given velocity. However, I prefer to take advantage of this fact by so reducing the free area between the tubesof a row that the velocity is increased to produce the maximum draft which is allowable at the maximum duty. Then I obtain a greater velocity between the tubes than 'is at all possible with the designs embracing round tubes. An increased velocity results in a nearly proportionate increase in the film heat transfer on that surface per square foot. If the increased velocity is obtained by placing the tubes closer together than would be the case me. similar design embracing round tubes, there is afurther increase of-heat-transferfor the same velocity due to reducing the mean hydraulic depth of the passage. Furthermore, the area of my tube, even without transverse fins in effective contact with the exterior circulating medium as furnace gases, can be made much greater and even twice the exterior area of the round tube of the same width, As a result of this an increased heat transfer is obtainable per lineal foot of tube, with an increased heat transfer per square foot, and a greater square footage per lineal foot of tube, not so many rows of tubes are required to accomplish a given heat effect. This of itself will reduce the draft necessary at a given velocity. 3

greatly increasing the amountrof surface and the heat transfer for a given volume of apparatus I achieve a great saving in space and in cost of headers and casing.

Another advantage of my symmetrical type of streamlined tube is that the gases or air may go through the bank in either direction with equally eflicient draft loss and heat transfer in either direction. In many types of air heaters or other heat exchange apparatus it is necessary to lengthen the path of the gases circulating around the tubes by dividing the bank with transverse bafiies so that the gas will go across the bank at one end in one direction, and then be reversed and go back across the other end in the other direction; or there may be several passes back and forth. With non-symmetrical type of streamlining there is much higher resistance in one direction than in the other. i

1. For use with heat exchangers and the like, a plurality of tubes arranged in staggered rela--v tion, the exterior walls of which, when taken in transverse section, are exteriorly bounded by reverse curves, the opposed faces of adjacent tubes defining gas passages, bounded by said curves, of substantially uniform width between opposed surfaces of adjacent tubes,-- the walls of said tubes being of substantially equal thickness, each suchtube having diametrically opposed exteriorly convex side portions and diametrically opposed exteriorly concave sided extensions.

2. For use with heat exchangers and the like, a plurality of tubes arranged in staggeredrelation, the exterior walls of which, when taken in transverse section, are exteriorly bounded by reverse curves, the opposed faces of adjacent tubes defining gas passages, bounded by :said curves, of substantially uniform width between opposed surfaces of adjacent tubes, the Walls of said tubes being of substantially equal thickness, each such tube having diametrically opposed exteriorly convex side portions and diametrically opposed exteriorly concave sided extensions, the exteriorly concave side portions of use with heat exchangers and the like, a seamless tube, the walls of which, when taken in transverse section, are exteriorly bounded by reverse curves, the walls of said tube being of substantially uniform thickness throughout, the inner face of the walls conforming to the exterior,

the ends of said tube being formed with portions generally circular in cross section, and of diflerent diameter than the tube, the circular portion at one end of the tube being of greater diameter than the maximum diameter of the non-circular portion of the tube.

5. As a new article of manufacture, a seamless tube formed with exteriorly convex diametrically opposed side portions, and having slightly concave sided longitudinal elongations along said tube between said convex side portions, the walls ofsaid tube being generally uniform in thickness, the interior of said tube being bounded throughout in cross section, by freely flowing reverse curves.

6. For use with heat exchangers andthe like, a plurality of tubes the walls of which are exteriorly and interiorly bounded, in transverse cross section, byfreely flowing reverse curves the opposed faces of adjacent tubes defining gas passages,

width between opposed surfaces of adjacent tubes, said tubes being provided with a plurality I of exterior transverse parallel fins, said fins be? ingin ready heat conducting relationship with thewalls of said'tubes. I

'7. For use with heat exchangers and the like,- a tube the walls of which are exteriorly and interiorly bounded, in transverse cross section, by freely flowing reverse curves, said tube being provided with a pluralityof exterior transverse parallel fins, said fins being in ready heat conducting relationship with the walls of said tube, said fins extending outwardly a substantially uniform distance from the gas swept portions of the tube walls. I

8. As a. new article of manufacture, a tube having sides which conform in transverse cross section to smooth reverse curves, said tube including two strips of sheet stock, each such strip forming one-half ,of the tube, said sheets being united along their opposed edges.

9. For use with heat exchangers and the like, a plurality of tubes arranged in staggered relation, saidtubes being streamlined in exterior transverse cross section, and a supporting structure, positioned intermediate the ends of said tubes, said supporting structure being in general parallelism with the line of flow about said tubes and conforming to the exterior faces of said tubes and substantially filling the space between tubes.

10. For use with heat exchangers and the like,-

ROGER STUART BROWN. 

